Plenum fan

ABSTRACT

An improved plenum fan for use in air handling systems. The inlet cone is first in line in the direction of airflow. It is attached to the back plate, which is in between the fan wheel and the inlet cone. The fan wheel mates with the back plate through a non-contacting labyrinth seal. The wheel inlet and outlet are both cone-shaped so that the air channels between the fan blades are tilted towards the direction of airflow. The back plate is positioned behind inlet cone to permit a pressure sensor to be mounted on the inlet side of the back plate and to permit a fixed pressure tap to be connected on the inlet cone. Thus, a short conduit may be connected to the low side of the differential pressure gauge or sensor without passing through the fan back plate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/215,854, filed on Jul. 21, 2016, which is a continuation of U.S.patent application Ser. No. 13/589,772, filed Aug. 20, 2012, whichclaims the benefit of U.S. Provisional Application No. 61/526,528, filedAug. 23, 2011, and the contents of these applications are incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention generally relates to fans used in air handling orair delivery equipment for heating, ventilation, and air conditioningsystems (HVAC).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art single width housed belt driven centrifugal fanwith no outlet duct connected and the inlet funnel removed to show thewheel inlet.

FIG. 2 is a prior art single width belt driven centrifugal plenum fan.

FIG. 3 is a prior art single width direct driven centrifugal plenum fanshown from the air inlet side and from the air outlet side. The supportstructure is removed to provide clarity for the fan construction. Thisfan is exemplary of the type that would be typically used in a prior artfan array.

FIG. 4 is a side elevation of the prior art plenum fan from FIG. 3installed in an air handling unit cabinet to show how the cabinetconnects to the back plate of the fan unit and separates the lowpressure or inlet from the high pressure or outlet side of the fan.

FIG. 5 is an improved direct driven plenum fan shown from the air inletside and the air outlet side. The support structure has been removed toprovide clarity for the fan construction. This fan is one preferredembodiment of this invention.

FIG. 6 is a side elevation of the improved plenum fan from FIG. 5installed in an air handling unit cabinet to show how the cabinetconnects to the back plate of the fan unit and separates the lowpressure or inlet side from the high pressure or outlet side of the fan.

FIG. 7 is a 2×3 plenum fan array shown from the air inlet side and theair outlet side. This drawing shows an array with the improved directdriven plenum fans.

FIG. 8 shows the flow patterns in a prior art single width housed beltdriven centrifugal fan with an outlet duct connected.

FIG. 9 is a section view of the fan in FIG. 4 showing the flow patternin a prior art direct driven single width single inlet plenum faninstalled in an air handling unit cabinet.

FIG. 10 is an enlargement of the section view of FIG. 9 to show theareas of turbulence and the air recirculation through the gap betweenthe rotating wheel and the inlet cone.

FIG. 11 is further enlargement of the section view in FIG. 10 detailingthe air recirculation through the gap between the rotating wheel and theinlet cone.

FIG. 12 is a section view of the fan in FIG. 5 showing the improved flowpattern in the improved fan of this invention.

FIG. 13 is an enlargement of the section view of FIG. 12.

FIG. 14 is a further enlargement of the section view of FIG. 13 showingthe details of the rotating labyrinth seal.

FIG. 15 is a detailed section view of a prior art plenum fan wheel andcone showing an alternate shape of the fan wheel inlet plate thatincorporates a curved section at the air entry opening.

FIG. 16 shows four graphs, one each plotting torque, power, voltage, andspeed verses the VSD output frequency.

FIG. 17 shows a graph that plots 2-pole, 4-pole, 6-pole, and 8-polemotors relative to increasing VSD output frequencies.

FIG. 18 shows a graph that plots 5, 7.5, 10 and 15 horsepower motorsrelative to increasing VSD output frequencies.

FIG. 19 is a table that shows the minimum nominal motor efficiencyrequired to be rated as a premium efficiency motor by NEMA MG1 for2-pole, 4-pole, 6-pole, and 8-pole motors and for 5, 7.5, 10 and 15horsepower motors.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

A typical plenum fan (200), shown in FIG. 2, is an un-housed centrifugalfan. It has a plurality of blades attached to a fan wheel (201) arrangedin a backwardly inclined fashion about the axis of rotation (208). Theblades are flat or airfoil shaped or curved single thickness bladesshaped to approximate an airfoil shape. The blades are sandwichedbetween an inlet plate, with an opening in the center, and a back platethat are parallel to each other and normal to the axis of rotation. Theplurality of fan blades, the inlet plate and the back plate are fastenedtogether to form the rotating fan wheel (201). A stationary funnelshaped inlet cone (202) is positioned to direct air into an opening inthe inlet plate in the same direction as the axis of rotation.

As the fan wheel rotates, the air that is in between the fan blades isforced out of the outer perimeter of the wheel by the centrifugal forcesgenerated. The exit of the air between the blades leaves a void of airthat is filled by pulling air through the inlet cone and into the wheelinlet plate. The air that enters the inlet cone accelerates as it passesthrough due to the funnel shape of the cone that reduces the area as itnears the rotating wheel along the axis of rotation. As the air leavesthe inlet cone and enters the rotating wheel through the inlet plate, itis forced to change flow direction from axial to radial and is furtheraccelerated as it passes through the fan blades.

By the time the air reaches the outer perimeter of the fan blades it ismoving at tremendous velocity with correspondingly high momentum energy.When the air passes past the outer perimeter of the fan blades it slowsdown and the momentum energy of the air is converted to pressure. Inthis way the plenum fan can move the air and create pressure that willforce the air through the air handling system of the building.

Early plenum fans were produced by simply removing the outer housings ofsingle width single inlet housed fans (100). The wheels were designed tooperate in a housing (101) that captured the air as it exited the wheeland directed it to a connection plate that allowed the fan assembly tobe attached to a duct. The housing was usually shaped as an expandingvolute around the wheel and it was designed to efficiently convert thehigh velocity air momentum energy into pressure.

Using the wheels designed for single width single inlet housed fans inplenum fan designs worked, but these plenum fans were less efficientthan the housed fans for which the wheels were originally designed. Theplenum fans could have up to ten percent (10%) less static efficiencythan the housed fans. Despite this loss in efficiency plenum fansgradually replaced housed fans in air handling unit applications. Plenumfans had many advantages over housed fans when used inside of airhandling units. They are quieter.

They have much lower downstream velocity due to the absence of thehousing. This lower velocity reduces the internal losses of the airhandling unit and reduces problems like water blow off on cooling coilslocated downstream from the fans. As the plenum fan designs gainedgreater acceptance, fan manufacturers improved the designs of thewheels. By concentrating on the aerodynamic shape and position of theblades, the design of the inlet cones and other key parameters of thefan wheel, plenum fans became more efficient and quieter. They cancurrently deliver air more efficiently than a comparably housedcentrifugal fan when mounted inside of an air handling unit. They havebecome the most common type of fan used inside of large commercial airhandling units for that reason.

During the same time period that large commercial air handling equipmenthas changed from predominantly using housed fans to predominantly usingplenum fans, it has become very common to power those fans with variablespeed drives. Variable speed drives (VSD) are electronic devices placedon the power line of three phase induction motors. They are able tocontrol the frequency and voltage of the electrical power delivered tothe motor. In that way they can control the rotational speed of themotor and the fan attached to it. They have become almost universal inusage on large commercial air handlers because they offer substantialbenefits to the owners of the air handling equipment and have beensteadily decreasing in price in recent years. They can substantiallyreduce the power consumption of the fans on variable air volume systems.They can compensate for filter loading. They can control buildingpressurization. They can offer many more benefits too numerous tomention here but well known to those skilled in the art.

Fans for large commercial air handling equipment and plenum fans inparticular have historically been designed to be driven by electricmotors through belt drive systems. When a manufacturer of plenum fansdesigned the fans they would generally create a line of geometricallysimilar fan wheels. A typical fan line may have wheels that range indiameter from 12 inches to over 90 inches. The fans in the line may becapable of air flow volume of 1000 cubic feet per minute (cfm) to over100,000 cfm at pressures ranging from 0 to over 12 inches water columnof static pressure. Due to manufacturing limitations there would be alimited number of discrete sizes in the line of fans. The fanmanufacturer would select the wheel size increments carefully so thatthe line would efficiently cover the range of flow and pressure targetedfor the fan line.

In order to apply a fan in an air handling system, a fan would beselected by a designer to deliver the flow and pressure he needs for hissystem. He would select the best size to fit his application anddetermine the fan speed and motor power required by that fan. Thedesigner would consider the fan efficiency, the cost, and the range ofoperation required when making his selection. He will select one of thefan sizes that will deliver stable operation over the range ofconditions he expects for the application. That selection will result ina design or maximum speed of rotation for the fan and require anelectric motor of sufficient size to meet the fan power consumption. Therotational speed requirement can vary widely over the fan line. It couldbe anywhere from 100 revolutions per minute (rpm) to 5000 rpm for fansin a typical fan line. By connecting the motor to the fan through a beltdrive, the fan manufacturer can satisfy this widely varying requirementfor fan speeds with common motors. He could simply select thecombination of belt drive pulley for the motor shaft and the fan shaftto give the fan rotational speed required. For the reasons stated abovebelt drives were the preferred method to drive plenum fans in largecommercial air handling systems.

When a belt driven fan is used in combination with a VSD, there are twodevices on the fan that affect the rotational speed of the fan.Designers realized that if a VSD was used, the belts could be eliminatedand the motor could be mounted directly to the fan. Direct drive fanshave significant benefits over belt driven fans. They are less expensiveto manufacture. They eliminate the drive belts, the pulleys, the fanshaft and bearings. The fan frame can also be smaller and lesscomplicated and less costly. Belts reduce the efficiency of the systembecause they contribute to frictional losses that can amount to 3% to 6%of the power consumed by the fan.

These frictional losses end up as heat that the belts must dissipate tothe atmosphere. Drive belts constantly wear because of this heat and theconstant rotational flexing and tension on the belt. As a result, beltsmust be changed quite often during the life of the fan. If they are notchanged, they will eventually break causing the air handling system tofail.

Direct drive fans have advantages, but the existing plenum fan linesdesigned for belt drives have some substantial limitations for this use.As stated earlier, these fans are designed to be geometrically similarand the designs typically follow similarity rules known as fan laws.These fan laws are well known to those skilled in the art. In general,as the fan diameter gets larger they are used to deliver more air. In afan line or family of fans the smallest to the largest fans are calledon to deliver a similar range of pressures. Larger fans must be turnedat a slower speed than smaller fans to deliver similar pressures. Thisleads to some problems with matching motors to fans for direct driveapplications. Since the motor shaft speed must equal the fan rotationalspeed in a direct drive fan, the motors must operate over a wider rangeof speeds than they would have to in a belt driven application.

Three phase induction motors are typically used to drive fans. Therotational speed of the motor is primarily determined by the frequencyof the electrical power delivered to it and the number of poles in themotor windings. This determines the rotational speed of the electricfield applied to the rotor of the motor. This rotational speed is thesynchronous speed of the motor. The rotor runs at a slightly slowerspeed than the synchronous speed in order to develop enough torque tomeet the power demand applied to it by the fan load. Motor manufacturerscommonly offer motors with 2 pole, 4 pole, 6 pole, or 8 pole windings.Motors in the US are usually designed to operate and deliver full ratedhorsepower at 60 Hz. At 60 Hz, this results in synchronous speeds of3600 rpm, 1800 rpm, 1200 rpm, and 900 rpm respectively. In Europe andother countries where electrical power is transmitted at 50 Hz, theresulting synchronous motor design speeds are 3000 rpm, 1500 rpm, 1000rpm, and 450 rpm respectively.

When the speed of the motor is controlled by a VSD, the motor candeliver full power in a limited range. Normally the motor will deliverfull power when the VSD delivers frequency from the motor's designfrequency to about 1.5 times the motor's design frequency. For example,a modern 3 phase high efficiency 4 pole motor designed to produce 10horsepower (HP) with 460 volt, 60 Hz power will deliver 10 HP when theVSD delivers frequency between 60 Hz to 90 Hz. Depending on the designof the motor the range of full power delivery will vary somewhat. Inthis range the VSD can deliver full voltage to the motor and the motorwill not overheat. When the VSD delivers a frequency below the motor'sdesign frequency, it must drop the voltage below the motor's designvoltage or the motor will overheat. In this range the maximum voltagethe drive can deliver and the maximum shaft power the motor can deliveris proportional to the delivered frequency. When the VSD deliversfrequency below the motor's design frequency, the motor's maximum poweroutput is reduced by the ratio of the delivered frequency to the motordesign frequency. The example 10 HP motor can only deliver 7.5 HP at 45Hz.

Because the larger fans run slower, it becomes more expensive to matchthe motor to the fan when the fans get larger. The more poles the motorhas the heavier and more expensive it is to manufacture for any givenshaft power delivery. That is, because more poles require more windingsand the slower a motor runs the more torque it must produce for anygiven shaft power output. It takes more motor mass to produce moretorque, which drives up the price of the motor. When the design speed ofthe fan is below the synchronous speed of an 8 pole motor, the motorwill have to be de-rated to apply them at that slow speed, furtherincreasing the cost and weight of the motor selection. In addition toproblems of increased motor cost and weight, the fans get exponentiallyheavier as they get larger. The largest fans can be too heavy for themotor bearings. These problems with matching the motors to the fans havelimited direct drive plenum fan applications to smaller fans andtherefore smaller airflows.

In order to expand the benefits of direct drive plenum fans to largerair handling systems, fan arrays have become popular in large commercialair handlers. Instead of one or two large fans in a system, fan arraysuse a larger number of smaller fans to deliver the same airflow. Arraysof plenum fans have been produced using as many as 30 to 40 direct drivefans in systems that deliver air volumes in excess of 200,000 cfm. Thesmaller fans run at faster speeds, overcoming the problems with heavyand costly low speed motors. In addition to the benefits of direct drivefans in large air handling equipment, fan arrays provide other benefitsas well.

Because the smaller fans used in fan arrays run faster to producecomparable pressure, they have a better sound power spectrum. The large,low speed fans produce very high sound power levels in the first threeoctave bands, and it is very difficult to remove noise in these octavebands with conventional sound attenuation techniques due to their longwavelength. Smaller fans running at higher speeds produce less noise inthe first three octave bands. They usually produce more noise in thefourth and fifth octave band, but it is much easier to attenuate noisein these bands.

The smaller fans used in fan arrays are shorter in the direction ofairflow which reduces the length required for the air handling system.This saves space in the building and reduces the cost of the casing ofthe air handler. The smaller motors used in fan arrays weigh less andare easier to replace in the event of a motor failure. There is moreredundancy with fan arrays increasing reliability. Small direct driveplenum fans used in fan arrays can be stacked horizontally andvertically, improving the airflow patterns of the air handler.

These many benefits are making air handling systems with fan arrays verypopular with institutions and other long term owners of buildings. Oncurrently designed plenum fan lines, the smallest of the fans tend to beless efficient than the larger fans. Using these smaller fans in fanarrays in place of the larger fans can reduce the efficiency of the airhandling system. This is a significant disadvantage of fan array systemsusing current plenum fan technology.

Turning to FIG. 5, which shows a preferred embodiment of this invention,we will describe the features of the current invention. FIG. 5 is anisometric drawing of the improved fan shown from the inlet and theoutlet side of the fan unit (400). The fan unit (400) consists of a fanback plate (401), a fan inlet cone (402), a rotating fan wheel (403),and a fan motor (404). As shown in FIG. 13, the fan unit (400) includesa fan inlet (450) that defines a circular converging-diverging air flowduct (451). The air flow duct (451) extends along an air flow centerline(452) and includes a throat (453). The air flow duct (451) is formedfrom the fan inlet cone (402) and a diverging air expansion portion(454). As shown in FIGS. 6 and 13, the fan inlet cone (402) is abell-mouthed converging air inlet portion. FIG. 6 shows this fan as itwould be installed in a housing (406) which shows how the low pressureor inlet side of the fan unit (400) is separated from the high pressureor outlet side by connecting the back plate (401) to the housing (406)with a flexible membrane (405). As shown in FIG. 13, a mount member(455) is connected to a discharge end of the fan inlet cone (402) formounting the fan unit (400) to a wall of the housing (406). The mountmember (455) includes the back plate (401), the diverging air expansionportion (454), and a non-contacting labyrinth seal (414). The back plate(401), also referred to as a radially extending wall, is orientedtransverse to the air flow centerline (452) for mounting the fan unit(400) to the wall of the housing (406). FIG. 13 also shows that the backplate (401) is positioned proximate to a discharge end of the divergingair expansion portion (454) and axially downstream of the throat (453).These diagrams are for illustrative purposes and do not show thesupporting structure of the fan unit (400), motor (404), and fan wheel(403). It would be understood by one skilled in the art that a supportstructure would be necessary for the proper function of the fan unit(400). The details of the support structure are not included in thedrawings to add clarity to the features of the present invention.

Comparing this new fan unit (400) to the prior art fan plenum fan unit(300) detailed in FIG. 3 and FIG. 4 you will notice several majordifferences.

The first significant difference is the location of the back plate (401)relative to the fan wheel (403) and the inlet cone (402). On the priorart plenum fan unit (300), the back plate (301) is first in line in thedirection of the airflow. The inlet cone (302) is then attached to theback plate (301). The rotating fan wheel (303) is then positioned withits inlet plate (308) inserted over the edge of the inlet cone (302)creating an air gap (317) better illustrated in the section viewdrawings of FIG. 9, FIG. 10, and FIG. 11.

The inlet cone (402) of the improved fan unit (400) is first in line inthe direction of airflow. It is attached to the back plate (401), whichis now in between the rotating wheel (403) and the inlet cone (402). Thefan wheel (403) is then positioned to mate with the back plate (401)through the non-contacting labyrinth seal (414) created by featuresformed into the back plate (401) and the fan wheel inlet plate (410)illustrated in the section view drawings of FIG. 12, FIG. 13, and FIG.14. The changed position of the back plate (401) is advantageous to thepositioning of the labyrinth seal (414) which will be discussed in moredetail later.

The second significant difference is the shape of the rotating fan wheel(403). The fan wheel (303) of the prior art plenum fan (300) consists ofa plurality of fan blades (309) positioned radially about the axis ofrotation and sandwiched between a flat inlet plate (308) and a flatoutlet plate (307). These fan blades are positioned in a backwardlyinclined position, as illustrated in FIG. 9. The wheel inlet plate (308)has an opening formed into it to allow the insertion of the inlet cone(302). The plurality of air channels (316) formed between the fan blades(309), the inlet plate (308), and the wheel outlet plate (307) directsthe air radially outward from the axis of rotation and perpendicular tothe direction of flow at the fan inlet. The back plate (307) is flat andis attached to the shaft of the motor (304) which drives the rotation ofthe fan wheel (303).

The shape of the fan wheel (403) of the current invention is markedlydifferent than that of the shape of the prior art fan wheel (303).Instead of being flat, both the wheel inlet plate (410) and the wheeloutlet plate (409) are formed in the shape of a cone. As shown in FIGS.13 and 14, each of the wheel inlet plate (410) and the wheel outletplate (409) includes a side wall that is disposed opposite of the otherside wall and is inclined at an angle away from the fan inlet (450). Aplurality of fan blades (411) is positioned radially about the axis ofrotation and is disposed or sandwiched between the cone-shaped inletplate (410) and cone-shaped outlet plate (409). The plurality of airchannels (417) formed between the fan blades (411), the wheel inlet(410) and the wheel outlet plate (409) are tilted forward into thedirection of airflow at the fan inlet by the angle of the cone shape ofthe wheel inlet (410) and wheel outlet plate (409). This forward tilt ofthe air channels (417) provides for improved flow which shortly will bediscussed in more detail. The outlet plate (409) is attached to theshaft of the motor (404) which drives the rotation of the fan wheel(403).

It should be noted that the diagrams in FIG. 3, FIG. 4, FIG. 9, FIG. 10,and FIG. 11 show the construction of a prior art plenum fan (300) with acompletely flat fan wheel inlet plate (308). This is the commonconstruction technique for most of the high efficiency plenum fans soldin the United States today. Other shapes of this inlet plate have beenused. Some prior art plenum fans (350) incorporate a curved shape (352)at the inner edge of the wheel inlet plate (354) as shown in the sectionview of FIG. 15. This curved shape (352) acts as a continuation of theshape of the inlet cone (351) and the fan blades (356) are usuallyformed to fit seamlessly along this curved shape. In most cases of priorart plenum fans the curved shape (352) eventually flattens out (353)creating radially outward flow channels (357) similar to those shown forthe prior art plenum fan (300) with a completely flat fan wheel inletplate (308). The tilted forward flow channels (417) of the currentinvention differ from this in that both the wheel inlet plate (410) andthe wheel outlet plate (409) are tilted forward to form forward flowingchannels (417) about the fan wheel blades (411). These forward flowingchannels (417) have advantages for a plenum fan that we will discussnow.

As stated earlier, plenum fans were developed by removing the housingsfrom housed single width single inlet centrifugal fans. Many fanmanufacturers still use the exact same fan wheel and inlet cone designfor their plenum fans that they use for their housed single width singleinlet (SWSI) centrifugal fans. The flow characteristics in the wheelchanged when the housing was removed. It is useful to explain how theflow was altered when the housing was removed to explain the benefits ofthe current invention.

FIG. 8 shows two section views of prior art SWSI fan (100) with flowlines drawn in to show how the air moves through the fan. Since thesefans are designed to be ducted and the outlet duct affects the fanperformance, the fan section is shown with an outlet duct attached. Theview on the right is a section view through the center of the fan wheel(102). The flow lines (114) enter the fan from a plenum space throughthe inlet cone (103). The air accelerates through the cone and reaches amaximum velocity at the minimum section of the inlet cone (103). Fromthere the air expands into the fan wheel (102) where it is pulledthrough the wheel by the centrifugal forces acting on the air in the airchambers formed between the fan blades (110) and the wheel inlet plate(108) and wheel outlet plate (109). The air is ejected into the housingat high velocity and is contained by the fan housing (101). Thecentrifugal action of the wheel changes the direction of the flow froman axial direction to a radial direction about the fan axis of rotation.Because of this direction change and the high momentum of the air as itpasses through the minimum section of the inlet cone, the flow of theair is not completely radial as it passes through the fan blades (110)as shown by the flow lines (114). Because the wheel outlet plate (109)is very close to the closed side of the housing (101), the air leavingthe wheel is forced to stay in a radial direction as it exits the fanwheel.

The left view is a section view through the fan wheel (102), fan housing(101), and outlet duct (111) looking at the wheel inlet. This view showshow the air interacts with the housing after it leaves the rotating fanwheel (102). The flow lines (115) show how the air passes through thewheel (102) and is collected by the fan housing (101) and forced intothe outlet duct (111). The housing (101) is shaped in an expandingvolute around the outer diameter of the fan wheel (102) in a way thatallows the air to gradually expand as it flows around the volute andinto the outlet duct (111). This gradual expansion allows the fan toconvert most of the momentum energy of the high velocity air exiting thefan wheel (102) from velocity pressure to static pressure as it isforced into the lower velocity outlet duct (111). This effectiveconversion from velocity energy to static pressure improves the staticefficiency of the fan which is why these fans are usually more efficientthan the corresponding prior art plenum fan (300).

In contrast, FIG. 9, FIG. 10, and FIG. 11 show the flow through a priorart plenum fan. FIG. 9 shows two section views of a prior art plenum fan(300). The left view shows a section through the middle of the fan wheel(303) looking at the wheel from the motor side. The flow lines (311) areall exiting the fan radially. Without a housing to direct the flow, theair exits in a rather uniform fashion and expands out in this directionto fill the limits of the housing (306).

The right view is a sectional view through the center of the fan wheel(303). The flow lines (310) enter the fan from the inlet plenum throughthe inlet cone (302). The air accelerates through the cone and reaches amaximum velocity at the minimum section of the inlet cone (302). Fromthere the air expands into the fan wheel (303,) where it is pulledthrough the wheel by the centrifugal forces acting on the air in the airchambers (316) formed between the fan blades (309) and the wheel inletplate (308) and wheel outlet plate (307). The air is ejected from thefan wheel (303) at high velocity where it rapidly diffuses into theoutlet plenum and loses velocity. This loss of velocity momentum is notgradual or well controlled, so most of the momentum energy or velocitypressure is lost by conversion to heat and results in lower than optimumstatic efficiency for the fan (300). Because there is no housing to keepthe air flowing in a radial direction and the air handling cabinet (306)directs the air to continue moving in a direction parallel to the axisof rotation, the tendency to shift the flow pattern in a forwarddirection as it passes through the fan blades (309) is accentuated. Thistendency toward forward flow is shown by the flow lines (310).

FIG. 10 is a more detailed view of the right section of FIG. 9 showingonly the flow through the upper half of the wheel. The view shows areasof turbulence caused by the less than uniform nature of the air flowingthrough the chambers formed by the area between the fan blades (309).The turbulence (313) as the air exits the wheel at the downstream sideof the wheel is great and is caused by the high exit velocity passingover that edge of the wheel in the forward direction. Turbulenceincreases the amount of noise the fan produces and reduces the fanefficiency. Reducing areas of turbulence will decrease fan noise andincrease fan efficiency, both desirable results. Other areas ofturbulence (312 & 314) exist in the voids caused by an absence of flowdue to the tendency towards forward flow. The angle of this forward flowfrom the plane normal to the axis of rotation of the fan wheel (303)ranges from 15 degrees to 35 degrees depending on the flow through thefan (300) and the fan wheel design.

FIG. 11 is an enlarged view of FIG. 10 showing the air bypass (315)through the gap (317) caused by the difference in diameter between theinner diameter of the fan wheel inlet plate (308) and the outer diameterof the inlet cone (302) at its exit. This gap is relatively small, butthe pressure differential between the air inside the fan and outside thefan is very great at this point. This velocity of the air on the insideof the fan is very near to its maximum value at this point. It can be ashigh as 10,000 to 20,000 feet per minute. The corresponding velocitypressures are 6 to 25 inches of water column respectively at standardatmospheric conditions. Since this velocity energy comes from convertingthe static pressure at the inlet of the fan to velocity pressure, thestatic pressure must be less than the pressure at the inlet of the fan(300) by at least those values. In other words, a pressure probemeasuring between the fan inlet and the air inside the fan at this airgap would measure negative 6 to 25 inches of water column. The purposeof the fan is to generate flow and positive pressure between the inletand the outlet. Plenum fans of this type can generate positive staticpressures that exceed 12 inches of water column. The pressure differenceacross this gap can be as large as 35 inches and is routinely in therange of 15 to 20 inches of static pressure. This gap is in the range of0.06 inches to 0.18 inches for fan wheels with diameters ranging from 12inches to 30 inches. These small gap sizes are like an open door whenexposed to these large pressure differentials. As much as 2% to 5% ofthe fan airflow can bleed back through this gap depending on theoperation conditions. The velocity jet passing through the gap is veryhigh and contributes to the turbulence (314) in the dead area on thisside of the wheel and contributes greatly to the noise generated by thefan during operation. The high bleed back air volume reduces theefficiency of the fan by an amount equal to the percentage it representsof the total air volume of the fan.

The improved plenum fan (400) of the present invention reduces theturbulence and air bypass of the prior art plenum fans (300). FIG. 12,FIG. 13, and FIG. 14 illustrate the improved flow patterns of thepreferred embodiment. FIG. 12 shows two section views of the preferredembodiment plenum fan (400). The left view shows a section through themiddle of the fan wheel (403) looking at the wheel from the motor side.The flow lines (412) are all exiting the fan radially. Without a housingto direct the flow, the air exits in a rather uniform fashion andexpands out in this direction to fill the limits of the housing (406) ina very similar fashion to the prior art plenum fan (300). This is adesirable feature because it promotes uniform air velocity downstream ofthe fan which gives plenum fans many advantages when used in airhandling unit applications.

The right view is a section through the center of the fan wheel (403).The improved flow lines (412) enter the fan from the inlet plenumthrough the inlet cone (402). The air accelerates through the cone andreaches a maximum velocity at the minimum section of the inlet cone(402). From there, the air expands into the fan wheel (403) where it ispulled through the wheel by the centrifugal forces acting on the air inthe air chambers (417) formed between the fan blades (411) and the wheelinlet plate (410) and wheel outlet plate (409). The air then passes intothe outlet diffuser space (418) formed by extending the wheel inletplate (410) and the wheel outlet plate (409) past the outside edge ofthe fan blades (411). In this space the air is allowed to expand in agradual and controlled manner and convert some of the velocity pressurelost by prior art plenum fans into static pressure and thereby improvethe efficiency of the fan.

The flow in the fan is improved because the air chamber (417) is formedby the shape of the conical fan wheel inlet plate (410), and the conicalfan wheel outlet plate (409) allows the air to flow smoothly in theforward direction that it wants to flow due to the momentum forcespreviously described. This shape eliminates the areas of high turbulenceby eliminating the sharp edge the air must pass at the exit andminimizing the areas void of airflow.

The air bypass and the efficiency loss and noise that result from it aregreatly reduced by the labyrinth seal (414) in the preferred embodimentof the present invention. This seal is created by the shape of astationary part or portion (415) formed in the stationary fan back plate(401) and the corresponding shape of a mating part or portion (416)formed on the rotating fan wheel inlet plate (410). As shown in FIG. 14,the stationary part (415) defines fingers or teeth (456) and the matingpart (416) defines fingers or teeth (457) that interlock with each otherin a manner that increases the flow path distance and resistance thatthe air must pass through to bypass from the fan outlet plenum to theinside of the wheel at the point of the rotating air gap. FIG. 14 showsthat each of the teeth (456) is formed from a respective axially- andforwardly-extending annular wall of the stationary part (415) and eachof the teeth (457) is formed from a respective axially- andrearwardly-extending annular wall. FIG. 14 also shows a series ofu-shaped concentric grooves (458) defined by the teeth (456) in such amanner that each of the grooves (458) is separated from another of thegrooves (458) by one of the teeth (456). A plurality of u-shapedconcentric grooves (459) are defined by the teeth (457) in such a mannerthat each of the grooves (459) is separated from another of the grooves(459) by one of the teeth (457). As the wheel rotates at high speed theteeth (457) of the mating part (416) that are formed into the fan wheelinlet plate slide in the grooves (458) of the stationary part (415)formed into the fan back plate (401) at great velocity. This velocityinduces air movement in the grooves that is perpendicular to thedirection that the bypass air must flow which further reduces the amountof air bypassed through this seal.

In the preferred embodiment of this invention the stationary part (415)of the labyrinth seal (414) is formed into the fan back plate (401)because it is convenient and cost effective to do so. That is one of thereasons why it is desirable to have the fan back plate (401) locatedafter the fan inlet cone (402) instead of before it as it is in priorart plenum fans (300). Additionally, as shown in FIG. 14, the divergingair expansion portion (454) extends from the stationary part or portion(415) of the labyrinth seal (414) and is connected to and downstreamfrom the fan inlet cone (402). It is not necessary for the stationarypart of the labyrinth seal to be in the fan back plate to be effective.Other embodiments may form this part into the inlet cone (402), and itwill function just as well.

Another reason to move the fan back plate (401) downstream of the inletcone and place it between the inlet cone and the rotating fan wheel asit is in the preferred embodiment is to reduce the installation cost ofpiezometric measurement of the fan air flow. Heinz Wieland, in his EUpatent application 90114296.8, detailed how a simple, highly accurateair flow measurement system could be created by installing a pressuretap at a fixed location along the contour of the inside surface of theinlet cone. By measuring the static pressure depression between the airentering the inlet cone and this pressure tap, the air volume flowingthrough the fan can be accurately determined by using the calculationmethods he disclosed.

This method of flow measurement has become very popular for use withplenum fans and particularly popular when the plenum fans are used infan arrays. An example fan array is shown in FIG. 7. In prior art plenumfans it is necessary to pipe at least one pressure port through the fanback plate (301) in order to make the measurement required to calculatethe flow. The pressure port at the fixed location on the inlet cone isformed by drilling a hole through the inlet cone and attaching by somemechanical means a tube or conduit to transmit the low pressure back tothe low side of a differential pressure gauge or sensor. The high sideof the differential pressure gauge or sensor must be connected to sensethe pressure of the air in the inlet plenum before it enters the inletcone. When the fan back plate (301) is located before the inlet cone(302) as it is in prior art plenum fans (300) it is necessary to port apressure tube or conduit through the fan back plate (301) to accomplishthis measurement. One could locate the differential pressure sensor inthe downstream plenum and connect the low pressure side to the pressuretap with a tube or conduit. The tube or conduit would have to beconnected to the high pressure port of the sensor or gauge and runthrough the fan back plate (301) in order to sense the upstreampressure.

Alternately, the differential pressure sensor or gauge could be locatedin the upstream plenum and the high pressure port left unconnected sothat it would sense the upstream plenum pressure before the air enteredthe inlet cone (302). In this case a conduit or tube would have to beconnected to the pressure tap on the inlet cone and to do so the conduitor tube would have to pass through the fan back plate (301) to make theconnection. So in either location of the pressure sensor or gauge a tubeor conduit would have to be routed through the fan back plate (301) ifit is located before the fan inlet cone (302) as it is in prior artplenum fans.

In contrast, by moving the fan back plate (401) after the fan inlet cone(402) as it is shown in the preferred embodiment of this invention thepressure sensor or gauge could be mounted on the entering air side ofthe fan back plate (401). The connection to the fixed pressure tap onthe inlet cone would also be on the entering air side of the fan backplate (401) so a simple and short tube or conduit could be connected tothe low side of the differential pressure gauge or sensor withoutpassing through the fan back plate (401). This greatly simplifies theinstallation of this device and reduces the cost. This is particularlyimportant in fan array applications of the present invention, because itis desirable to put flow measurement on all of the fans in the fan arrayto get the most accurate flow measurements. Small cost savings on eachindividual fan can add up to large cost savings for the entire fanarray. Accurate flow measurement is increasingly important to implementmodern building energy savings strategies.

As stated earlier, fan arrays, such as the one shown in FIG. 7, havebrought the benefits of direct drive fan systems to large industrialsystems. As such they almost always employ direct drive fans and alsoalways use variable speed drives (VSD) to control the speed of themotors. The VSD is not only used to control the airflow of thesesystems, it is also typically used to set the design or maximum speed aswell. In prior art systems the motor is usually selected to run at aspeed higher than its synchronous speed in order to achieve the designor maximum performance.

FIG. 16 shows how the fan motor and the VSD work together. FIG. 16 showsthe performance parameters of a 4 pole, 3 phase induction motor rated at10 HP, 460 Volts and 1760 rpm.

The amount of torque a motor delivers is a function of the load imposedon it. The torque that any given motor delivers is a function of themotor slip and the volts/hertz ratio provided by the power supply, whichin this example is a VSD. The volts/hertz ratio is an importantoperating parameter of the motor that will be discussed in detailshortly.

The slip is the difference between the rotating field of the motor andthe actual rotor speed of the motor. The rotating field speed is oftencalled the synchronous speed of the motor. The synchronous speed of amotor is a function of the AC power frequency and the number of polesbuilt into the motor and can be calculated with the following formula:

Synchronous Speed=120×Power Frequency/number of poles in the motor

The power frequency is in hertz and the Synchronous Speed is in rpm. Inthe case of our example motor, the synchronous speed is 1800 rpm at itsdesign condition of 60 hertz. Since the motor is rated to deliver itsdesign power at 1760 rpm the slip is then 40 rpm. The motor is typicallyrated in horsepower, not torque but the two are related as shown by thefollowing formula:

Motor Power Output=Torque×Motor Speed/5252

The Motor Power Output is in horsepower (HP). The motor speed is in rpmand the torque is in lb-ft. For our example unit, the motor rated torqueoutput is 29.84 lb-ft.

If the load on the example motor is exactly 10 HP at 1760 rpm, the motorwill turn at 1760 rpm. If the load on the example motor requires lessthan 10 HP at 1760 rpm, the motor speed will increase (the slip willdecrease) until the torque output of the motor matches the load. If theload requires more than 10 HP, the motor speed will slow down (the slipwill increase) until the motor produces enough torque to meet the load.This would result in an overloaded condition on the motor, and thewinding temperature would increase and the motor current would riseabove the rated full load amps (FLA). For these reasons, overloading themotor is not recommended. It does point out that slip is an indicationof motor loading and that the slip is small, relative to the motorsynchronous speed, on modern three phase induction motors. This makesthe speed of the motor nearly proportional to the output frequency ofthe VSD.

There are 4 separate graphs on FIG. 16. There are graphs of Torque,Power, Voltage, and Speed verses the VSD output frequency. The graphsare broken into two regions, the “Constant Torque Output” region and the“Constant Power Output” region.

From 0 hertz to the full motor frequency rating, the VSD will adjustvoltage to the motor as its output frequency rises. That is from 0 hertzto 60 hertz for the example motor. The VSD will not allow thevolts/hertz ratio to exceed the design ratio of the motor. In the caseof the example motor, the volts/hertz ratio is 7.67. This means that ata VSD output frequency of 30 hertz, the drive output voltage will notexceed 230 VAC. This is important because if the motor sees highervoltage than that it will saturate, overheat and eventually fail. If thedrive delivers a constant volts/hertz ratio as it varies the outputfrequency from the motor design frequency down to 0 hertz, the motorwill be capable of delivering its full design torque at any point inthis range. This is called the “Constant Torque Output” region. It isimportant to note that, if the motor load requires less than the designtorque of the motor, the voltage can be decreased by the VSD. This isoften done by special algorithms in the VSD to improve motor efficiencyat low loads. This is especially important with fans because theirtorque requirements fall off exponentially as the fan speed is lowered.

The Torque curve of FIG. 16 has two lines: the first is the rated motortorque line, and the second is the maximum motor torque line. Themaximum torque line is a function of the motor design, and is generallya multiple of the motor rated torque. Both of these torque values areconstant as long as the drive maintains the constant volts/hertz ratio.When the VSD hits the motor rated frequency, it must deliver the ratedvoltage to the motor to keep the volts/hertz ratio constant. The wayVSD's are customarily applied with motors, the rated voltage of themotor is also the line voltage input to the VSD. The VSD does not havethe capability to deliver voltage to the motor in excess of the linevoltage. Consequently, if the frequency delivered to the motor exceedsthe motor design frequency, the volts/hertz ratio drops. As statedearlier, when the volts/hertz ratio to the motor is lower than thetorque, capability of the motor lowers. Fortunately, when the motor runsfaster than the design speed, it requires less torque to supply therated power of the motor. As seen from the torque curves, the maximummotor torque drops off faster than the torque required for constantpower. The point where these two curves cross is at the maximumfrequency the motor can deliver the full rated power. The frequencyrange between the motor rated frequency and this frequency is called the“Constant Power Output” region.

The maximum frequency varies on modern premium efficiency 60 hertzmotors. It will range from 90 hertz to 110 hertz depending on theoverall motor torque capability.

The power curve shows that the maximum motor power output variesproportionally in the constant torque region and is constant in theconstant power region. The motor can operate safely as long as itdelivers less power than bounded by these curves.

The voltage curve shows the motor voltage varies proportionally with thefrequency in the constant torque output region. This is what allows themotor to deliver constant torque. In this region, the voltagerepresented by the curve is also the maximum voltage that the VSD cansupply to the motor. The motor current draw in this region will be afunction of the torque required by the load. At full design torque, themotor will draw its rated full load current when the volts/hertz ratiois constant. If the load requires less torque the motor will draw lesscurrent. At low torque loads, the VSD will often decrease the voltage tothe motor below the constant volts/hertz line. This keeps the motorcurrent up and reduces the magnetic core loss, which increases the motorefficiency at low loads. In the constant power region, the curve showsthat the voltage remains constant at the maximum voltage output of thedrive. In this region, the motor current draw will be a function of thepower required by the load. When the motor delivers full ratedhorsepower in this region, it will draw its rated full load current. Atlower power demand, it will draw less current.

The speed curve shows that the motor speed varies proportionally withfrequency when it delivers rated torque in the constant torque region.It continues to rise at a slightly lower slope in the constant powerregion where it delivers constant power. The slope is slightly lowerbecause the motor needs more slip to deliver the necessary torque inthis region.

Prior art direct drive fans used in fan arrays are commonly selected sothe motors run in the constant power region at the full load or designcondition of the fan. This is because the motor does not need to bede-rated when operating in this region. If the motor is selected to runin the constant torque region it might take a larger motor to meet thepower demand. This would add additional expense for the motor andassociated electrical service. The motor would be heavier making itharder to spring isolate.

Fans are always selected at the maximum required flow and pressure forthe application it will be used for. As long as the motor is selected sothat the fan power draw at the required fan speed is within theoperating envelope of the motor it will remain within that envelope. Thepower curve of FIG. 16 shows a typical operating envelope of a directdrive fan motor. In this example the fan requires 9.0 bhp at 2250 rpm.This corresponds to a motor frequency of 80.0 hertz. As the power curveshows, this operating point is well within the power capability of the10 horsepower motor of our example. The VSD would be set up so that themaximum frequency output is 80 hertz. In that way the power required todrive the fan will never exceed the design power draw. The hatched areashows the potential operating range that the fan could impose on themotor. It is bounded on the lower edge by a curve that varies with thecube of the fan speed. This represents the minimum load that this fanmight impose on the example motor. This would simulate a system wherethe air distribution system had a fixed resistance to airflow. In thiscase the pressure rise of the fan would be reduced by the square of theratio of the operating speed to full load speed and the flow would bereduced by the direct ratio of those speeds. The operating range isbounded on the top by a straight line between the power required at thedesign point and the zero power and zero hertz point. This wouldsimulate a system where the pressure was constant and the flow reducedby the direct ration of operating speed to the full load speed. Actualsystems will be in the hatched area because the system resistancenormally is not completely fixed and the fans cannot deliver fullpressure as the speed drops off. Filters in the airflow path load up andtheir resistance to flow varies over time. Variable resistance devicesare often installed in the air distribution system to control flow tospecific zones. An example of a variable resistance device would be avariable air volume control box which is quite commonly used with modernair handling systems.

As stated earlier, motors for prior art direct drive fans are oftenselected in the constant power region of the motor as explained above.Both the VSD and the motor work well in this area, and the motor candeliver its full design power to the fan. It does not have to bede-rated as it would if the fan selection required the motor to run atless than its rated speed. If the motor were selected in the constanttorque region it would have to be de-rated. A larger more expensivemotor would have to be supplied. This larger motor would weigh more andrequire large electrical service to operate.

There are some difficulties that often prevent designers from selectingmotors to operate in the constant horsepower region and that add costeven at times when they can. The design fan speed requirement of a fancan vary over a large range and is driven by the flow and pressurerequired and the geometry of the fan selected. The range of selectionspeeds for common prior art plenum fans used in fan arrays varies from400 to over 4000 rpm. There is a limited selection of motors availableto the designer of the fan system. Only 2, 4, 6, and 8 pole three phaseinduction motors are commonly available for this application. Thosecorrespond to motors with synchronous speeds of 3600, 1800, 1200, and900 rpm at 60 hertz. FIG. 17 illustrates how the speed of these motorsvaries with the VSD output frequency and shows some of the problemsdesigners have with their motor selections. The speeds between the motorpole options do not overlap in the desirable constant power region.There is a gap between 2650 rpm and 3550 rpm in this example. That gapis caused because the maximum speed of the 4 pole motor is 2650 and theminimum speed of the 2 pole motor is 3550 in the desirable constanthorsepower region. If a fan needs to operate between these two speedsthe designer must select a larger 2 pole motor de-rated and operating inthe constant torque region. This gap can be reduced by producing the 4pole motor with much more than normal torque which would increase itsmaximum speed but this would cause additional expense in the manufactureof the 4 pole motor. Another gap exists below 850 rpm where the designermust select and de-rate an 8 pole motor.

Other problems exist when selecting motors to drive fans in this manner.Four pole motors are the most efficient motors to use. FIG. 19 shows theminimum nominal motor efficiency required to be rated as a premiumefficiency motor by NEMA MG1. The 2 pole motors are substantially lessefficient than the 4 pole motors for any given motor size. This isprimarily because the mechanical geometry of 4 pole motors is betterthan 2 pole motors. 4 pole motors are by far the most common of the 3phase induction motors. They account for over 80% of the motors used.The efficiency of four pole motors is more important than 2, 6, or 8pole motors to motor manufacturers and motor users so it is natural thatthe standard for the efficiency of 4 pole motors is a little morestringent than for the rest. 6 and 8 pole motors are uncommon, and theyrequire a lot more torque for any give power rating. As a result theyare much bigger and much more expensive than 4 pole motors.

The motor (404) used in the preferred embodiment of this invention isdesigned in a novel way to overcome many of the problems with using suchmotors to directly drive a fan.

Four pole motors are the most desirable motor to directly drive a fanbecause of their lower cost and higher efficiency than other commonlyavailable motors with 2, 6, or 8 poles. The motor (404) used in thepreferred embodiment of this invention uses a four pole motor with theelectrical windings in the stator designed to allow it to be used overthe entire selection from 850 to nearly 4000 rpm.

The torque that a 4 pole motor can produce is primarily a function ofthe mechanical design of the motor. The amount and size of the steellaminations in the stator and rotor and the copper conductor paths inthe rotor all have an influence on the amount of torque produced. Theultimate speed of the motor is also a function of the mechanical designof the motor. The bearings must be sized to withstand the speedsexpected by the motor and the rotor must be strong enough to withstandthe force exerted on it at its highest. The voltage rating of the motoris a function of the electrical design of the windings in the stator.The mechanical design and electrical design are separate elements ofmotor design. It is common for a single mechanical design to haveseveral electrical designs associated with it. Our example 10 horsepowermotor was designed for 460 volt 60 hertz power giving it a designvolts/hertz ratio of 7.67. If it were supplied with electrical windingsin the stator for 230 volt 60 hertz power the design volts/hertz ratiowould be 3.83. It is quite common for a motor mechanical design to haveseveral different electrical designs so that it can run on the commonvoltage distribution networks around the country. This characteristic ofelectric motor manufacture is used by the motor (404) of the preferredembodiment to make 4 pole motors that can be selected in their constanthorsepower region over a much wider range of speeds than available withprior art motor selections.

The motor manufacturer has the flexibility to design the volts/hertzratio over a wide range of values. The prior art motor uses avolts/hertz ratio of 7.67 as explained above.

When used on a VFD with 460 volt supply power, the maximum voltageoutput of the VFD is 460 volts, and the product of the maximum outputvoltage times the volts/hertz ration defines the minimum frequency thatfull power can be delivered at, which is also the lowest frequency inthe constant power region. The upper bound of the constant power regionis at a minimum 1.5 times the lowest frequency or in this case 90 hertz.

With electrical windings designed for 5.11 volts/hertz the minimumfrequency to deliver full power is 460 volts times 5.11 or 90 hertz.This would correspond to a rotational speed of around 2650 rpm. Theupper bound of the constant power region would still be at least 1.5times the lowest frequency or in this case 135 hertz. This motor couldthen deliver full power from 2650 rpm to at least 3700 rpm. Thiseliminates the gap that the prior art selection methods leave between2650 rpm and 3550 rpm. This selection method has an additionaladvantage. A motor with a mechanical design that delivers enough torqueto produce 10 horsepower at 60 hertz will be able to produce 15horsepower at 90 hertz. This is because the torque capability is afunction of the mechanical design and would be the same for both cases.Since power is proportional to torque times speed, the higher speedmotor is capable of producing 50% more power at 50% more speed. Sincethe cost of the motor would not be significantly changed by differentelectrical designs, the cost per horsepower will decrease byapproximately 33%. This is a significant reduction in cost while stillmaintaining the high efficiency of the 4 pole 10 horsepower motor.

Similarly, lower speeds can be addressed by increasing the volts/hertzratio. FIG. 18 shows the speed output of a 4 pole motor verses VSDfrequency delivery to the motor. The volts/hertz ratios shown are forVSDs on a 460 volt power distribution system, and the dark lines showthe frequency verses speed range for the constant power selection regionof each motor winding option. The motors designed with electricalwindings for the lower speeds would have the disadvantage that theywould deliver less power than the higher speed motors. As shown in FIG.17, the motor with windings designed for 10.22 volts/hertz could onlydeliver 7.5 horsepower between 1350 to 2000 rpm. Similarly, the motorwith windings designed for 15.33 volts/hertz can only deliver 5horsepower between 850 to 1400 rpm. Though these lower speed 4 polemotors would cost more per horsepower than the 10 horsepower motor, theywill cost less than equal power 6 and 8 pole motors with the same rangeof constant power speeds. They will also have similar efficiency to the10 horsepower motor which is substantially better than the equivalent 6and 8 pole motors.

While the example was given for a motor with a mechanical design of 10horsepower at 1750 rpm, this method of matching motor mechanical andelectrical designs with the speed ranges necessary to match to specificfan design criteria will work equally well with other motor sizes, powersupply voltages and motor pole designs. The preferred embodiment uses 4pole motors because they currently offer the best performance at thelowest cost. By using this method, cost effective, highly efficient 4pole motors can be selected with current VSD technology to drive fansfrom 850 to over 4000 rpm in their desirable constant horsepower range.This can be done with no gaps in the selection range.

Prior art centrifugal fans are designed to be built in families of fans.The fan manufacturer would design a range of fans that are similar ingeometry. These similarities are designed to constrain all of the majordesign dimensions of the fans to a ratio of the outer diameter of thefan wheel. The major design dimensions would include, but not be limitedto, the fan blade (309) length, the dimension between the wheel inletplate (308) and the wheel outlet plate (307), and the length and alldiameters of the inlet cone (302). The ratio of all of these dimensionsto the wheel's outer diameter would be constant throughout the fan line.The fan manufacturer would also decide several distinct wheel diametersto build for his line of fans. These sizes would be selected to provideoverlap and continuity of performance over the range of airflow andpressure the fan line is to cover. By doing so, the performance of allof the fans can be accurately predicted from testing done on only aselect few of the fans. The performance predictions can be projectedfrom test data by a set of similarity equations commonly known as theFan Laws by those skilled in the art. This method is commonly used todesign all modern centrifugal fan lines available today.

A problem with confining the fan geometry by these similarities existswhen trying to apply centrifugal fans at low pressures. Centrifugalplenum fans, which are the subject of the current invention, aretypically used to supply pressures that range from 1 inch Water Column(WC) of static pressure to over 12 inches WC of static pressure. Theycan cover this range quite effectively, but for any given airflow ittakes a larger diameter fan to efficiently deliver air at lowerpressures. This is because for any given fan size the fan must runslower to deliver less pressure and when it runs slower it delivers lessairflow than it would at lower pressures. So in order to find anefficient fan at low pressure the designer must pick a larger fan thanhe would at higher pressures and run it very slowly. This larger fan ismore expensive, and in the case of a direct dive fan, the motor getsmuch more expensive as has been previously discussed. Fans used for fanarrays are usually only provided in a relatively small range of sizesand models. Larger fans might not be available so more fan units mightbe required which will increase cost and space requirements. Often whenfaced with these alternatives designers choose to run the fans at lessefficient areas of their operating range. Those skilled in the art knowthis as running the fan down the fan curve toward the wide open volumepoint. A centrifugal fan has 0% static efficiency at wide open volumeand has its maximum or peak efficiency at somewhere around 50% wide openvolume. When a fan is selected to operate closer to wide open, staticefficiency is reduced.

One embodiment of the current invention addresses this problem andallows smaller fans to be used at more efficient operating points thanprior art centrifugal fans for low pressures. By shortening the lengthof the fan blades in the fan wheel you effectively reduce the diameterof the fan wheel. The pressure a fan develops is directly proportionalto the tip speed of the fan blades. By reducing the diameter of the fanwheel you will reduce the pressure it will develop at any given speed.If no other dimension of the wheel is changed the features that affectair volume flowing through the wheel are unchanged. These unchangedfeatures would include the swept volume of the fan blades at their airinlet point, the spacing and pitch of the blades, the distance betweenthe wheel inlet plate and the wheel back plate, and the shape and sizeof the inlet cone. By keeping these features constant while reducing thewheel diameter you end up with a wheel that produces more flow at lowpressures and requires higher speeds. All of these are advantages forfans used for direct dive applications at low pressures. Keeping thewheel speed up allows the designer to select higher speed motors whichare less costly for any given power rating. Higher wheel speeds alsoproduce less noise in the difficult to attenuate lower frequency bands.This is very similar to the practice of trimming the diameter of adirect coupled centrifugal pump wheel in order to achieve the designhead at a specific flow when the pump is run at a specific speed. Itdiffers from that in the aspect that the wheels to be trimmed are fanwheels and the purpose is to increase the speed and flow of the fan atlow pressure.

The embodiments shown and described above are exemplary. Many detailsare often found in the art and, therefore, many such details are neithershown nor described herein. It is not claimed that all of the details,parts, elements, or steps described and shown were invented herein. Eventhough numerous characteristics and advantages of the present inventionshave been described in the drawings and accompanying text, thedescription is illustrative only. Changes may be made in the details,especially in matters of shape, size, and arrangement of the partswithin the principles of the inventions to the full extent indicated bythe broad meaning of the terms of the attached claims. The descriptionand drawings of the specific embodiments herein do not point out what aninfringement of this patent would be, but rather provide an example ofhow to use and make the invention. Likewise, the abstract is neitherintended to define the invention, which is measured by the claims, noris it intended to be limiting as to the scope of the invention in anyway. Rather, the limits of the invention and the bounds of the patentprotection are measured by and defined in the following claims.

What is claimed is:
 1. A plenum fan unit for an air handler, comprising:a fan inlet defining a circular converging-diverging air flow ductcomprising a centerline and a throat, the fan inlet comprisingconverging air inlet portion extending to and terminating at the throat;a mount member connected to a discharge end of the converging air inletportion for mounting the plenum fan unit to the air handler, the mountmember including a radially extending wall oriented transverse to thecenterline for mounting the plenum fan unit to the air handler, whereinthe radially extending wall is positioned axially downstream of thethroat, a stationary portion of a radially-oriented labyrinth sealextending inwardly toward the centerline from the radially extendingwall, and a diverging air expansion portion connected to and downstreamof the converging air inlet portion, the diverging air expansion portionextending inwardly toward the centerline from the stationary portion ofthe radially-oriented labyrinth seal and terminating at the throat; afan comprising a plurality of fan blades disposed between opposed sidewalls that are inclined at an angle away from the fan inlet, wherein oneof the opposed side walls comprises a mating portion of theradially-oriented labyrinth seal that is configured to rotate when thefan rotates; and a motor connected via direct drive to the fan to rotatethe fan.
 2. The plenum fan unit of claim 1, wherein the radiallyextending wall extends to form a square periphery for connecting to theair handler.
 3. The plenum fan unit of claim 1, wherein the radiallyextending wall includes a flexible membrane extending therefrom along aperiphery of the radially extending wall to close any air gaps betweenthe radially extending wall and the air handler.
 4. The plenum fan unitof claim 1, including an annular receiver positioned at an inlet end ofthe diverging air expansion portion, wherein an inner diameter of theinlet end of the diverging air expansion portion mates with an outerdiameter of the discharge end of the converging air inlet portion. 5.The plenum fan unit of claim 1, wherein the motor is an induction motorand rotational speed of the motor is configured to be controlled by avariable speed drive (VSD).
 6. The plenum fan unit of claim 1, whereinthe motor comprises a 2-pole three phase induction motor, a 4-pole threephase induction motor, a 6-pole three phase induction motor, or an8-pole three phase induction motor.
 1. The plenum fan unit of claim 1,wherein the motor comprises a stator configured with windings to allowthe motor to operate over a range from 850 rpm to 4000 rpm.
 8. Theplenum fan unit of claim 1, wherein the stationary portion includes aplurality of labyrinth seal teeth that are received by a correspondingplurality of u-shaped concentric grooves of the mating portion.
 9. Theplenum fan unit of claim 1, wherein the plenum fan unit is configured asan array of plenum fan units for the air handler.
 10. A plenum fan unitfor an air handler, comprising: a fan inlet defining a circularconverging-diverging air flow duct comprising a centerline and a throat,the fan inlet comprising a converging air inlet portion extending to andterminating at the throat; a mount member connected to a discharge endof the converging air inlet portion for mounting the plenum fan unit tothe air handler, the mount member including a diverging air expansionportion connected to and extending downstream from the converging airinlet portion, the diverging air expansion portion extending downstreamof the throat and away from the centerline, a stationary portion of alabyrinth seal extending radially from a downstream end of the divergingair expansion portion, and a radially extending wall oriented transverseto the centerline for mounting the plenum fan unit to the air handler,wherein the radially extending wall extends radially away from adownstream end of the stationary portion of the labyrinth seal, whereinthe radially extending wall is positioned axially downstream of thethroat, a fan comprising a plurality of fan blades disposed betweenopposed side walls that are inclined at an angle away from the faninlet; and a motor connected via direct drive to the fan to rotate thefan.
 11. The plenum fan unit of claim 10, wherein the stationary portioncomprises a series of u-shaped concentric grooves arranged proximate toone another.
 12. The plenum fan unit of claim 11, wherein the stationaryportion comprises a plurality of rearwardly-extending annular walls,wherein each of the rearwardly-extending annular walls defines astationary labyrinth seal tooth and wherein each of therearwardly-extending annular walls extends between adjacent u-shapedconcentric grooves of the stationary portion.
 13. The plenum fan unit ofclaim 11, wherein one of the opposed side walls comprises a matingportion of the labyrinth seal that is configured to rotate when the fanrotates, wherein the mating portion comprises a plurality of u-shapedconcentric grooves arranged proximate to one another and in a staggeredrelationship with the series of u-shaped concentric grooves of thestationary portion.
 14. The plenum fan unit of claim 13, wherein themating portion of the labyrinth seal comprises a plurality offorwardly-extending annular walls, wherein each of the forwardlyextending annular walls define a rotating labyrinth seal tooth.
 15. Theplenum fan unit of claim 10, wherein the motor comprises a statorconfigured with windings to allow the motor to operate over a range from850 rpm to 4000 rpm.
 16. The plenum fan unit of claim 10, wherein theplenum fan unit is configured as an array of plenum fan units for theair handler.
 17. The plenum fan unit of claim 10, wherein the fancomprises an axial inlet and a centrifugal discharge for air flow. 18.The plenum fan unit of claim 10, wherein the motor comprises a 2-polethree phase induction motor, a 4-pole three phase induction motor, a6-pole three phase induction motor, or an 8-pole three phase inductionmotor.
 19. The plenum fan unit of claim 10, wherein the fan inletcomprises a bell-mouthed shape.
 20. The plenum fan unit of claim 10,including an annular receiver positioned at an inlet end of thediverging air expansion portion, wherein an inner diameter of theannular receiver mates with an outer diameter of the discharge end ofthe converging air inlet portion.